Process for producing single crystal metallic alloy objects



D United States Patent 11 13,536,121

[72] Inventor Barry J. Piearcey [56] References Cited Brixlwm, Enzlmd UNITED STATES PATENTS P 3,366,362 1/1 968 Chandley et al. 164/60x [22] Flled Nov. 25, 1969 3,373,795 3/1968 Hem 164/361X D1v1s1on of Ser. No. 540,114, Feb. 17, I966,

3,376,915 4/1968 Chandley l64/60X now Pat. No. 3,494,709, and Ser. No.

3,401,738 9/1968 Parllle l64/36IX 764,171,0ct. 1,1968,wh1ch are contmua- 3 405 220 10/1968 B t (M/361x tions-in-part ofSer. N0. 459,391 May 27 e 1965 abandoned 3,411,563 11/1968 Fleck 164/127X .e. 3,417, l2 4s] Patented 0a. 27, 1970 809 1968 Smk [64/127 [73] A i U it d Aimrafl Corporation Pnmary Exammer-J. Spencer Overholser East H tf d Connecticut Assistant Examiner-.lohn E Roethel AttorneyMorgan, Finnegan, Durham and Pine a corporation of Delaware [54] PROCESS FOR PRODUCING SINGLE CRYSTAL METALLIC ALLOY OBJECTS 7 Claims, 9 Drawing Figs.

52 us. 01 164/60, 164/ l 27 [51] Int. Cl 822d 25/06 [50] Field of Search 164/60,

l27,338,353,36l; 148/16; 23/(Yudkoff, no search); 415/ 1 77. 2l6; 416/241 ABSTRACT: A method is provided for casting a single crystal metallic alloy object of complex shape formed from a heat-resistant and corrosion-resistant alloy having a face-centered cubic crystal structure, said crystal being oriented with its direction less than 20from the elongated axis of the crystal, said single crystal object having laterally enlarged integral base and generally being in the form of a gas turbine blade or vane.

Patented Oct. 27, 1970 I 3,536,121

Sheet 1 of 3 F/G F/a 3 5469/ /a af/ 5r W FTZZ/VZ/ Patehted Oct. 27, 1970 Sheet PROCESS FOR PRODUCING SINGLE CRYSTAL METALLIC ALLOY OBJECTS The application is a division of my prior applications Ser. No. 540,114 filed Feb. 17, 1966 now US. Pat. No. 3,494,709 and Ser. No. 764,171 filed Oct. 1, 1968 which are continuations in part of my application Ser. No. 459,391 filed May 27, 1965 and now abandoned.

The present invention relates to a novel and improved process for the formation of elongated shaped objects comprising a single crystal oriented in a particularly desirable direction and to an apparatus useful in carrying out the process.

The present invention has for its object the provision of a novel and improved process for the formation of shaped objects, such as an elongated blade or vane for a gas turbine engine formed as a single specially oriented crystal.

While the process of the present invention is of wide usefulness in the formation of single crystal objects of relatively complex shape, having the crystal axis in a predetermined relation to the shape of the object, the single crystal objects, especially the blades and vanes suitable for use in a gas turbine engine, are most usefully formed from nickel-base super-alloys, especially those alloys which are commercially known as Mar M-200 and most preferably from Mar M-200 which is substantially free of both boron and zirconium and with extremely low carbon content.

According to the present invention, the preferred and illustrative apparatus for carrying out the process to form the cast single crystal objects having a crystalline orientation substantially parallel to the length of the cast object comprises a ceramic shell mold, usually formed by the lost wax method" which rests upon a thermally conductive surface, preferably adapted to be water cooled, which shell mold is adapted to be radiantly heated by an inductively heated graphite susceptor, so that it may be brought to substantially the temperature of the melting point of the alloy to be cast, or slightly higher initial temperature at the upper part of the mold, while the water-cooled support remains substantially below the melting point, to facilitate solidification of the alloy to be cast.

Such a mold comprises an enlarged base cavity, which is connected to one or more shaped mold cavities by inclined Chromium 14. 2-15. 8 13. -15. 0 8. 0-11. 0 14. 0-16 0 8. 0-10. 0 Cobalt 14. 3-15. 1 1.0 13. 0-17. 0 17. 0-20. 0 9. 11. 0 'Iungsten 11. 5-13. 5 Molybdenum 3. 3-4. 0 3 .1-5. 5 2. 0-1. 0 4 5-5. 5

Columbium 2 1. 0-3. 0 0. 75-1. 25 Aluminum- 4. 6-5. 4 5. 5-6. 5 5. 0-6. 0 3 75-4. 75 4. 75-5. 25 Titanium" 3. 5-4. 3 0. 75-1. 25 4. 5-5.0 2. 75-3. 75 1. 75-2. 25

0 10-0. 20 0. .2 0 -0. 0. 03-0. 1 0 09-0. 17 Copper 0. 1 1 0. 5 0. 1 0. 10 Manganese I 0.015 1 1. l 0. l 0. 15 1 0. 20 Sulfur I 0.015 0. 015 0. 015 0. 015 0. 015 Silicon 0. 2 1. 0 0. 2 0. 2 1 0.- 2- Nickel (essentlally) Balance Balance Balance Balance Balance metal in the lower enlarged central portion of the shell mold, on cooling, crystallizes and grows more rapidly along the axis,

the lower portion of the constricted portion of the shell mold becomes filled with oriented columnar crystalline alloy which tends to grow sidewise and upwardly, and gradually induces the molten alloy filling the shaped portion of the molds to solidify as a single oriented crystal having its direction substantially coinciding with the elongated or principal stress axis ofthe object being cast.

The casting operation is preferably carried out in vacuum, or in an inert atmosphere, preferably argon, although for less demanding uses, the casting operation may be carried out in all.

After casting, the cast objects may be heat treated to improve their mechanical and physical properties, before or after which the lower portion of the casting and the sprue may be cut away, and any necessary machining may be done on the cast objects.

While the process is of wide usefulness with many different metals, alloys and other substances which crystallize on cooling, the blades and vanes to be used in a gas turbine, according to the present invention are alloys having compositions falling within the following weight percent ranges:

Percent Chroniumnflufl. 2-25 Cobalt 4-30 A1nminum Up to 9 Titanium Up to 6.0 Molybdenum and tuugstvn" 2-14 Carbon Up to 0.5 Boron Up to 0.1 Zirconium Up to 0.2

the balance of the alloy being essentially nickel in an amount of at least 35 percent.

Alloys which are especially adapted for use with the present invention and are preferred have the following elements in the weight percentage ranges set forth below, it being understood that copper, manganese, sulfur, and silicon are generally considered the impurities.

01 lllt following alloys, No. 659 is the preferred range.

PWA Alloy Number 1 1 Maximum.

2 Plus Tn.

Even better results are obtained where the quantities of boron and zirconium are further reduced, and most preferably the boron is present in a maximum quantity of less than 0.001

. percent by weight, while the zirconium has a maximum of 0.01

percent by weight, and Alloy PWA No. 659 is especially benefited by reduction of the boron and zirconium content to these very low limits.

While the usual minor amounts of boron and zirconium are they are supported on a water-cooled supporting member, thel highly advantageous in objects made of PWA Alloy No. 659

(Mar-M200) which have a conventionalheterogeneous, equiaxed crystalline structure, the presence of boron and zirconipart made from the alloy.

. member for a gas turbine produced in accordance with the The boron. and zirconium content of the alloy in small areas,

in .the .dendritic. structureof the crystal, thus producing 3 weakeningdiscontinuities in the structurewith a consequent impairment of all of the various propertiesof the single crystal. 1

blade or same. In :the utilization of the present invention,1

where the bladesandvvanesare in the form of single crystals,

the presence of minute dispersed concentrations of boron or,

be eliminated so far as commercially; feasible.

zirconium within the crystal is a distinct disadvantage and'is to v Due to the factthat all commercial raw materials are impure, and that it iscommercially impractical to obtain a pure. raw material, such relatively pure: materials are used, care being taken that they. do not introduceexcessive amounts of impurities. In the practiceof thepresent invention, foroptimum results, care is taken that the raw materials are substantially free of boron and zirconiurn and that the raw materials are melted in proper crucibles which introduce no excessive amounts of impuritiespMagnesia .crncibles are ordinarily avoided as they area frequent source of boron contamination. r

Zirconia crucibles are a frequent sourceiof zirconium. Alu- V mina and aluminum silicate crucibles arepreferred as they 3 may'beobtained substantially free of boron andzirconium contaminating constitutents.

- f All of the. nickel base," heat and corrosion resistantalloys.

such as PWA Nos. l0l IA 655, 658, 659 and 689 are benefi- 4 FIG. 9' is a perspective view of an illustrative form of a vane present invention.

I Referring now in detail to the present preferred and illustrarive mold for use withthe present invention, a simple form of inold for. carrying outthe process of theipresent invention is shown in FIGS. 1 and 2, in which .there is provided a shell mold having aninterior shape appropriate to the object to be'molded. This mold comprisesa relatively thin-walled shell which has preferablybeen formed by shell molding technique for use in the lost-Iwax method of casting, and is to be used in a u relatively high vacuum, less preferablyin an inert atmosphere of argon or helium, or sometimes in an atmosphere ofair.

The mold 20 is formed to rest on a relatively cool, heat conductive, and preferably water-cooled block 22, which is conveniently made of a relatively thick piece of copper or copper alloy. The block during the casting process is maintained ata temperature considerably below .the solidification temperature of the alloy orother material to be csst. I r

The lower portion of the mold 201comprises a relatively wide cavity 24 which communicates with a restricted passageway-26 connecting the basecavity 24 with the mold cavity proper 28. Thepassageway26may be of circular cross section, as shown in FIG. 2,or may be otherwise shaped, butis nonlinear and hasa relatively small cross section compared with the cross section of the lower portion. and is preferably M upwardly inclined to communicate with .the mold cavity 28.

a refractory material, in accordance with standard shell-moldcialin thesingle crystalcondition byv reduction of the boron and zirconium content. as specified above.

Thelimit of 0.00ljpercent for boron and 0.0l percent for zirconium isbased upon the type. of analytical procedure. used. 1

ing techniques. r r h 65.3 and 4illustrate an improved and preferable form of molding apparatus. In this form, the shell mold 30 is formed to .extending k and preferably upwardly inclined. nonlinear The prefsenceof boron and zirconium in the alloy can be con- 'firmed by analytical; methods, but the analytical procedure necessary to providequantitative values below the valuesof 0.001 percent boron and.0.0l percent zirconium was not at: 1

tempted in this instance. For this reasoman accurate, precise maximum content or these elements has not been established, 1 "except as herein specified. V V 7 h Theprocess of the invention is useful in connection with the 1- production of single crystal cast objects from a widevariety of y alloys forming face-centered crystals.

According tothe present invention, the resulting cast objects are almost always in. the form of single crystals properly oriented with respect to the stress axis of thecast object.

Of the drawings:

for use with the presentinvention.

vention.

FIG. I is a schematic vertical sectional view through a mold r FIG. 4 is a cross-sectional view taken'on the line 4-4 of FlG. 1

passageway 34 which leads to a second restricted laterally ex tending passage 36, which preferably extends in a different direction, and communicates with the mold portion proper 38; Mold 30 is open at the top to receivethe molten metal from which theohject is to be molded, and rests upon a relatively cool and preferably water-cooled copper block 22 which establishes a temperature gradient within the molten metal filling the mold, so that solidification ofgthealloy within the mold begins at the bottom of the mold.

' 1 As; shown in FIG. 4,. the restrictedpassageway 34 is preferably a relatively narrow slot, and the portion 36 is similarly shaped, to assist in insuring that thesolidified metal within the mold portion proper 38 is in the; form ofa single crystal, the, crystal axis extending lengthwise of the mold portion 38, that is, in a substantially vertical direction.

7 FIG. 5 =isa schematic vertical section showing a further; modification of the mold especially adapted forthe casting of v a plurality of elongated objects each formed of a single crystal.

of metal or other. material. 7

FIG. 6 is a vertical section showing a casting apparatus for use in accordance with jthe present invention together with FIG. 7 is a perspectiveview of an illustrative form of rotor in accordance with the FIG. 5 illustrates a form of. molding apparatus in which a plurality of mold cavities 50 are connected with a single base cavity 52 resting on a copper cooling block 22. Each of the cavities 50 is connectcdwith the base cavity by means of a 5 restricted, laterally and upwardly. extending passageway. 56..

Attheir. upper ends, mold cavities 50 are connectedwith a central mouth 58 through which the molten metal is introduced to fill the several parts of the molding apparatus.

FIG. 6 of the drawings illustrates schematically and in. a more complete manner a molding apparatus used for carrying out the process of the present invention. The entire apparatus j shown in FIG. 6 is preferably enclosed within a vacuum chamber (not shown) or within a chamber which maybe filled l with argon or other inert gas. The mold portion 60 provides an gends showing the several materials and the sum; of m imolded object. i h

enlarged base cavity6l, abovewhich is theportion 62 providing an upwardly and laterally extendingrestricted passageway 63 communicating with the mold cavity 65 formed by the shell .64. Above the mold cavity 65 is the pouring mouth 66 formed by theuppermost portion of the shelLand within which the sprue 67 forms;

mold to the desiredtemperatur eforcasting. Preferably, the shell mold is surrounded by a graphite susceptor 70, and this conventional slurry of alumina or other high-meltingpoint provide 'a base cavity 32 which communicates with a laterally Surrounding the shell mold arevthe means for heating the r in turn is surrounded by an induction coil 72 supplied with high frequency electric current as is usual in a high frequency induction furnace. Prior to casting, the shell mold is seated on the cooling block 22, the chamber is evacuated or filled with inert gas and the coil 72 is supplied with current to heat the shell mold to the desired temperature for casting. When the desired temperature has been attained. the molten metal, heated to the proper temperature for casting, is poured into the mold mouth 66 to fill the mold, the copper chill block being maintained relatively cool so as to establish a temperature gradient within the molten metal filling the mold as the metal solidifies. Power is shut off from the coil 72, and the assembly is allowed to cool.

' After completion of the process of the present invention, the shell mold and cast metal are removed from the furnace, and the shell mold is broken away from the cast object, after which surplus metal is machined away to provide the cast object, after which the surplus metal is machined away to provide the cast blade or vane member formed by the mold cavity 65.

The metal within the base cavity 61, when the metal is a face-centered cubic crystalline alloy, has a controlled columnar crystalline structure, with the crystals extending upwardly within the base portion and into the restricted passageway 63. With the restricted passageway 63, the solidified metal becomes a single crystal which fills the mold cavity 65. the crystal axis extending substantially vertically along the length of the blade or vane member. This single crystal structure extends into the mouth 66 of the shell mold, and the sprue portion 67 generally exhibits an uncontrolled polycrystalline growth.

FIG. 7 of the drawings illustrates a rotor blade for use in a gas turbine, which blade is of conventional shape, but is differentiated from the rotor blades of the prior art by being formed of a single crystal of a face-centered cubic crystalline alloy, the single crystal having a orientation with respect to the elongated axis of the blade member. Exact coincidence between the direction of crystallization of the single crystal forming the blade member and the longitudinal axis of the blade member is not essential and as much as a 20 deviation between the crystal direction and the longitudinal axis is acceptable, it being understood that the closer the crystal direction and the longitudinal axis coincide, the more fully the principle objects of the present invention are achieved.

As shown in FIG. 7, the rotor blade member comprises a root member 80, a shroud portion 82 and an intermediate airfoil portion 84, all portions of which are formed as a single crystal of a face-centered cubic crystalline alloy, having the composition of the broad range of nickel base alloys set forth above, and most preferably having a composition set forth with respect to the alloy designated as PWA 65 9 (Mar-M200 Airfoil gas turbine members which are to be subjected to internal cooling during operation may be provided with an internal passage or passages through which a cooling fluid is circulated during operation of the turbine. Such a blade is shown in section in FIG. 8, the blade otherwise being shaped in accordance with that shown in H0. 7. In FIG. 8, the airfoil section, root and shroud portions are formed with a smooth internal passage or passages, and as shown the passage 86 is formed in the blade. Like the blade of FIG. 7, the blade of FIG. 8 having a longitudinally extending interior passage 86 is formed as a single crystal of a face-centered cubic crystalline alloy having its orientation substantially coinciding with the longitudinal axis of the blade member.

FIG. 9 of the drawings illustrates a conventional form of vane member 88 for use as an airfoil member in a gas turbine, and which is formed as a single crystal of a face-centered cubic crystalline alloy in which the direction substantially coincides with the principal longitudinal axis of the vane member.

The process of the present invention is illustratively described especially with respect to the apparatus shown in FIG. 6 of the drawings:

A shell mold having a mold cavity 65 of the desired shape, and enlarged base cavity 61, a laterally and upwardly directed restricted passageway communicating between the base cavity 61 and the mold cavity 65 and provided at its top with an enlarged, upwardly extending mouth 66 is firmly seated on a copper chill block 22 within a vacuum induction furnace. The shell mold is preheated by current supplied to the induction coil 72, thereby heating the susceptor element 70 and the shell mold itself. The shell mold at its lower end is maintained at a lower temperature by means of the copper chill block 22 which is cooled by means of water circulating in a lower portion of the block 22.

The shell mold is preferably heated to a temperature of about 2600 F. for the casting of PWA 659, and the temperature of the upper face of the chill block 22 is preferably maintained at a temperature of not more than 200 F.

The interior of the furnace is either evacuated to a pressure of 10- mm. (Hg) or less, or is purged and filled with an inert gas, preferably argon.

A suitable quantity of the alloy to be cast, such as PWA 659, is then melted within the furnace by high frequency inductive heating, and when the molten alloy has been heated to a temperature above its melting point, preferably to a temperature of about 2,600 F., the alloy is poured into the mold so as to completely fill the mold.

The molten alloy immediately begins to solidify at its lower portion within the base cavity 61 where the molten alloy is in contact with the cool, chill plate 22. Initially there is formed a very thin layer of uncontrolled polycrystalline solidified alloy on the surface of the chill block 22. These uncontrolled crystals having a haphazard orientation give way to the more rapid upward growth of the crystals so that in the upper portion of the base cavity 61 the crystals are substantially all of orientation. As the crystal growth proceeds upwardly through the cooling mass of metal in the mold. a few of the upwardly growing crystals having an orientation enter the restricted laterally and upwardly directed passageway 63 and one crystal continues to grow laterally and then upwardly into the mold cavity 65, and the growth in the major portion of the restricted passageway 63 and completely in the mold cavity 64 is a single crystal of the face-centered cubic crystalline alloy.

During the solidification of the alloy heat is continually drawn away by the water-cooled copper chill block 22 so that a temperature gradient is always maintained between the bottom portion and the upper portion of the metal within the shell mold.

After the casting and solidification of the elongated object within the shell mold has been completed and has cooled to a moderate temperature at which the single crystal cast part is no longer subject to deleterious action by exposure to air, the chamber may be opened to break the vacuum or to allow air to enter the furnace chamber, and the shell mold and its enclosed cast part may be removed from the furnace. When the shell mold and part have cooled, the shell mold may be broken away from the cast part, and the cast part is then ready for machining to accurately finish its root and shroud portions, and for any finishing which may be required on the airfoil section, although such machining of the airfoil section is generally not required.

Test specimens of face-centered cubic crystalline alloy parts, blades and vanes produced in accordance with the present invention exhibit surprisingly superior properties compared with uncontrolled polycrystalline and directionally solidified parts of the same alloys, as fully disclosed in my copending U.S. Pat. application Ser. No. 540,119 filed Feb. 17, 1966, now U.S. Pat. No. 3,494,709.

I claim:

1. A method of casting a complex part having longitudinal dimensions substantially greater than any of its transverse dimensions and being formed of a single crystal of a strong heat resistant face-centered cubic alloy, said crystal being oriented with its direction approximating the longitudinal axis of the part, which comprises; positioning a mold having a large base portion on a cooled supporting member and an upwardly inclined restricted passagewayhaving its upper end laterally.

offset withrespect to the bottom end of the passageway connecting the base portion with an upperportion of the mold,

heating the upper portion of the mold to maintaina tempera. I ture gradient from the chilled plate to theupper part of the .1

mold and pouring and casting a face-centered cubic crystallizable alloy in the mold as a single crystal complex part. I 1

2. A method as claimed in claim 1 in which the mold comprises a large .baseportion resting on the chilled plate and the upper portions of the mold are initially maintained at a tern-- perature above the melting point oi the alloy while the lower portion is below the meltingpoint of the alloy. 3. A method as .c laimed in claim! inwhich a base portion. and a top portion of the mold cavity are relatively wideand short with respect to the'intermediate connecting portion. r

4. A method as claimed in claim 3 in which saidpart is in the form of a gas turbine blade. I r 5. Amethod as claimed in claim 3 in which form of a gas turbine vane.

said part is" in thef:

e 6. A method as claimed in claim 1 in which the alloy has 5;.

.Zirconium Up to 0.2

from 2-14 percent of metal selected from the group consisting of molybdenum and tungsten, and the balance of the alloy consisting essentially of nickel in an amount of at least 35 percent. I

7; A method as claimed in claim 2 in whichthe restricted passageway connecting the base portion :of the moldwith its upper portion is a nonlinear passageway.

-19c (ERA-1211C) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Paten 3,536,121 Dated October 27, 1970 Inventor-(p) Barry J. Piearcey It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the abstract, line 4, after "its" insert [001] In Column 1, line 30, sfter "a"; Column 2, line 4, after "with"; Column 2, line'S", after "single" and after "its"; Column 5, line 27, after "the"; Column 5, line 37, after "a"; Column 5. line 39, after "the" and before "direction"; Column 5, line 43, after "the" and before "crystal"; Column 5, line 65, after "its"; Column 5, line 70 after "the"; Column 6, line 33, after "the and before "crystals";

Column 6, lines 34 and 35, after "of" and before "orientation" Column 6, line 37, after "an"; and Column 6, line 74, after "its", insert:

Anew

Edward M. Fletcher, Ir. mm B. sum, m- L nesting Offim Gonnissioner of Patents-l 

